Power storage device

ABSTRACT

A power storage device which has improved performance such as higher discharge capacity and in which deterioration due to peeling or the like of an active material layer is less likely to be caused is provided. In an electrode for the power storage device, phosphorus-doped amorphous silicon is used for the active material layer over a current collector as a material that can be alloyed with lithium, and niobium oxide is deposited over the active material layer as a layer containing niobium. Accordingly, the capacity of the power storage device can be increased and the cycle characteristics and the charge-discharge efficiency can be improved.

TECHNICAL FIELD

The present invention relates to power storage devices.

Note that the power storage device indicates all elements and deviceswhich have a function of storing power.

BACKGROUND ART

In recent years, a variety of power storage devices such as lithiumsecondary batteries, lithium ion capacitors, and air cells have beendeveloped. In particular, a lithium secondary battery in which chargeand discharge are performed by transfer of lithium ions between apositive electrode and a negative electrode has been attractingattention as a secondary battery with high output and high energydensity.

An electrode for a power storage device is manufactured by forming anactive material layer over one surface of a current collector. Theactive material layer is formed using an active material such as carbonor silicon, which can store and release ions behaving as carriers. Forexample, when an active material layer is formed using silicon orphosphorus-doped silicon, the theoretical capacity is higher than thatin the case where the active material layer is formed using carbon,which is advantageous in increasing the capacity of a power storagedevice (e.g., Patent Document 1).

However, it is known that the volume of silicon serving as an activematerial is expanded when silicon occludes lithium ion and contractedwhen silicon releases lithium ion. Therefore, a problem arises in thatan active material layer is powdered and peeled from a current collectoralong with charge and discharge of a battery, for example. As a result,the current collecting property in an electrode is decreased and thecharge-discharge cycle characteristics are degraded. As a countermeasureagainst this, there is a method in which a surface of an active materiallayer is coated with carbon, copper, nickel, or the like to suppresspowdering and peeling of silicon; however, such coating may suppressreaction between lithium and silicon and may reduce the charge-dischargecapacity.

REFERENCE Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.    2001-210315

DISCLOSURE OF INVENTION

An object of one embodiment of the present invention is to provide apower storage device which has higher charge-discharge capacity andimproved performance such as higher cycle characteristics.

In an electrode for the power storage device, a material that can bealloyed with lithium is used for an active material layer over a currentcollector, and a layer containing niobium is formed over the activematerial layer, whereby the capacity of the power storage device can beincreased and the cycle characteristics and the charge-dischargeefficiency can be improved.

The layer containing niobium is preferably formed using niobium oxide orniobium nitride. Further, the layer containing niobium may contain aniobium-lithium alloy such as Li₂Nb₂O₅. In addition, the layercontaining niobium may be amorphous or crystalline.

Li₂Nb₂O₅ is formed by reaction between Nb₂O₅ and Li due to batteryreaction. In charge and discharge thereafter, the Li₂Nb₂O₅ may be held,or Li may be desorbed from the Li₂Nb₂O₅ so that Nb₂O₅ is formed. Thus,the Li₂Nb₂O₅ formed over the active material layer functions as a stableinorganic solid electrolyte interface (SEI) instead of an organic SEI,thereby having effects of reduction in resistance, improvement inlithium diffusivity, suppression in volume expansion of the activematerial layer, and the like.

As the active material, a material that can be alloyed with lithium ispreferably used; for example, a material containing silicon, tin,aluminum, or germanium can be used. Further, it is preferable thatphosphorus or boron be added to the active material. With the use ofsuch a material, the capacity of the power storage device can beincreased.

The crystallinity of the active material may be any of the following:amorphous, microcrystalline polycrystalline, and single crystal.Further, in the case of using silicon as the active material, forexample, the active material layer can include a crystalline siliconregion and a whisker-like crystalline silicon region which has aplurality of protrusions over the crystalline silicon region.Furthermore, a structure in which amorphous silicon exists aroundcrystalline silicon may be employed. The whisker-like crystallinesilicon region may include a protrusion having a bending or branchingportion.

In the above, a crystalline silicon layer including the whisker-likecrystalline silicon region can be formed over the current collector by athermal chemical vapor deposition (CVD) method, a low pressure chemicalvapor deposition (LPCVD) method, or a plasma CVD method, in whichdeposition is performed using a deposition gas containing silicon.

In this manner, the crystalline silicon region and the whisker-likecrystalline silicon region which has a plurality of protrusions over thecrystalline silicon region are provided in the active material layer,whereby the surface area of the active material is increased. In thepower storage device, when the surface area of the active material isincreased, the amount per unit mass of carrier ion such as lithium ionwhich is occluded by the active material per unit time or the amount perunit mass of carrier ion which is released from the active material perunit time is increased. The amount of carrier ion occluded per unit timeor the amount of carrier ion released per unit time is increased, andthus the amount of carrier ion occluded or released at a high currentdensity is increased; therefore, the discharge capacity or chargecapacity of the power storage device can be increased.

For the current collector, a material with high conductivity can beused, such as a metal element typified by platinum, aluminum, or copper.Alternatively, the current collector may be formed using a metal elementthat forms silicide by reacting with silicon.

In the power storage device, an electrolyte layer which is formedbetween a negative electrode and a positive electrode facing thenegative electrode can be formed using a liquid or a solid, and theelectrolyte layer may contain niobium.

In one embodiment of the present invention, a multi-layer structureincluding a current collector, an active material layer, a layercontaining niobium, and the like can be employed, whereby substancesconstituting the current collector, the active material layer, and thelayer containing niobium are bonded to each other and thus the strengthof the structure can be increased. Therefore, structural damage due tochange in the volume of the active material layer along with charge anddischarge is less likely to be caused. As a result, even aftercharge-discharge cycles, damage to the active material layer issuppressed; accordingly, increase in the resistance inside a battery anddecrease in the capacity can be suppressed.

In one embodiment of the present invention, an electrode of a powerstorage device can be formed by a coating method. For example, a slurryin which silicon particles are mixed as an active material is appliedover a current collector and then baked to form a coated electrode, anda layer containing niobium is formed over the coated electrode; thus, acoated electrode having high capacity and favorable cyclecharacteristics can be formed.

In one embodiment of the present invention, a material containingniobium can be used as an additive for the coated electrode used in thepower storage device.

According to one embodiment of the present invention, thecharge-discharge efficiency is improved and thus constant voltage (CV)charging becomes unnecessary. Consequently, the charging time isshortened and the cycle characteristics of a negative electrode materialcan be improved.

According to one embodiment of the present invention, a power storagedevice having improved battery performance such as higher dischargecapacity or charge capacity can be provided. A power storage device inwhich deterioration due to peeling or the like of an active materiallayer in an electrode is suppressed can be provided.

BRIEF DESCRIPTION OF DRAWINGS

In the accompanying drawings:

FIG. 1 illustrates an electrode of a power storage device;

FIGS. 2A and 2B illustrate a manufacturing process of an electrode of apower storage device;

FIG. 3 illustrates an electrode of a power storage device;

FIGS. 4A and 4B illustrate a manufacturing process of an electrode of apower storage device;

FIGS. 5A and 5B are a plan view and a cross-sectional view,respectively, which illustrate one embodiment of a power storage device;

FIGS. 6A to 6D are perspective views illustrating application examplesof a power storage device;

FIG. 7 illustrates an example of a structure of a wireless power feedingsystem;

FIG. 8 illustrates an example of a structure of a wireless power feedingsystem;

FIG. 9 illustrates a manufacturing process of a power storage device;and

FIG. 10 shows battery characteristics of a power storage device.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments and an example of the present invention will bedescribed with reference to the drawings. Note that the presentinvention is not limited to the following description and it will beeasily understood by those skilled in the art that modes and details canbe modified in various ways without departing from the spirit and scopeof the present invention. Therefore, the present invention should not beconstrued as being limited to the following description of theembodiments and the example. In description referring to the drawings,in some cases, the same reference numerals are used in common for thesame portions in different drawings. Further, in some cases, the samehatching patterns are applied to similar parts, and the similar partsare not necessarily designated by reference numerals.

Embodiment 1

In this embodiment, an electrode of a power storage device which is oneembodiment of the present invention and a method for manufacturing theelectrode will be described with reference to FIG. 1 and FIGS. 2A and2B.

FIG. 1 illustrates one embodiment of an electrode of a power storagedevice. The electrode of the power storage device in FIG. 1 includes acurrent collector 101, an active material layer 103 provided over onesurface of the current collector 101, and a layer containing niobium 109provided over the active material layer 103.

The current collector 101 is tanned as appropriate using a conductivematerial which can be used for a negative electrode current collectorand has heat resistance high enough to withstand heat treatment to beperformed later. Examples of the conductive material which can be usedfor the current collector include, but are not limited to, copper,platinum, aluminum, nickel, tungsten, molybdenum, titanium, and iron.Note that in the case of using aluminum for the current collector, analuminum alloy to which an element that improves heat resistance, suchas silicon, titanium, neodymium, scandium, or molybdenum, is added ispreferably used. Alternatively, an alloy of any of the above conductivematerials may be used.

Alternatively, the current collector 101 may be formed using a metalelement that forms silicide by reacting with silicon. Examples of themetal element that forms silicide by reacting with silicon includezirconium, titanium, hafnium, vanadium, niobium, tantalum, chromium,molybdenum, tungsten, cobalt, and nickel.

Alternatively, an oxide conductive material can be used for the currentcollector 101. Typical examples of the oxide conductive material includeindium oxide containing tungsten oxide, indium zinc oxide containingtungsten oxide, indium oxide containing titanium oxide, indium tin oxidecontaining titanium oxide, indium tin oxide, indium zinc oxide, andindium tin oxide to which silicon oxide is added. Note that the currentcollector 101 may have a foil shape, a plate shape, or a net shape. Withsuch a shape, the current collector 101 can hold its shape by itself,and a supporting substrate or the like is therefore not essential.

The active material layer 103 is preferably formed using a material thatcan be alloyed with an element whose ion carries electric charge. Theion which carries electric charge may be an ion of an alkali metal suchas lithium or sodium; an ion of an alkaline earth metal such as calcium,strontium, or barium; a beryllium ion; a magnesium ion; or the like, andlithium is preferably used. As examples of a material that can bealloyed with lithium, which can be used for the active material layer103, silicon, tin, aluminum, and germanium are given.

In the case of using silicon for the active material layer, a siliconlayer can be formed over the current collector 101 by a plasma CVDmethod or the like. In this case, it is preferable that a source gascontain hydrogen as little as possible in forming the silicon layer.Thus, defects formed in silicon, such as dangling bonds, can beincreased and insertion/extraction reaction of lithium ions can beeasily caused.

The layer containing niobium 109 provided over the active material layer103 can be formed using niobium oxide or niobium nitride. Instead ofusing niobium, an oxide or nitride of vanadium, tantalum, tungsten,zirconium, molybdenum, hafnium, chromium, or titanium can be used. Inaddition, the crystallinity of the layer containing niobium 109 may beany of an amorphous structure, a polycrystalline structure, and a singlecrystal structure.

Next, a method for forming the above electrode will be described withreference to FIGS. 2A and 213.

First, as illustrated in FIG. 2A, the active material layer 103 isformed over the current collector 101. For example, a titanium sheet maybe used as the current collector 101, and a silicon layer may be formedas the active material layer 103 over the current collector 101 by aplasma CVD method. The silicon layer may contain an impurity elementthat generates a carrier, such as phosphorus or boron. For example, inorder to make phosphorus be contained in the silicon layer, phosphinemay be contained in a source gas. Note that there is no particularlimitation on the crystallinity of the silicon layer. The silicon layermay be amorphous or crystalline. For example, amorphous silicon,microcrystalline silicon, or polycrystalline silicon can be used. Here,crystallization may be performed on the silicon layer. In the case wherethe silicon layer is subjected to crystallization, after the hydrogenconcentration in the silicon layer is sufficiently reduced, the siliconlayer may be crystallized by being subjected to heat treatment or laserirradiation.

Since the theoretical capacity of silicon is higher than that ofgraphite, an active material layer formed using silicon can have thesame level of capacity as an active material layer formed using graphiteeven when the thickness of the active material layer formed usingsilicon is approximately 1/10 of that of the active material layerformed using graphite. For that reason, the weight and size of asecondary battery can be reduced; however, when the thickness of theactive material layer is too small, the capacity of the secondarybattery is decreased. Thus, the active material layer 103 is formed to athickness greater than or equal to 50 nm and less than or equal to 10μm, preferably greater than or equal to 100 nm and less than or equal to5 μm. Even when the active material layer 103 is not formed to be thin,the capacity of the secondary battery can be increased by using silicon,which is preferable.

Next, as illustrated in FIG. 2B, the layer containing niobium 109 isformed over the active material layer 103. For example, a niobium oxidelayer may be formed as the layer containing niobium 109. The niobiumoxide layer can be formed by an evaporation method or the like with theuse of a Nb₂O₅ target. Alternatively, the niobium oxide layer may beformed by a plating method, a thermal spraying method, a CVD method, asputtering method, or the like.

The niobium oxide layer is preferably formed to a thickness greater thanor equal to 1 nm and less than or equal to 1000 nm, further preferablygreater than or equal to 80 nm and less than or equal to 500 nm. Inaddition, the composition of the formed niobium oxide layer can beexpressed as Nb_(x)O_(y) (x and y are each a positive integer).

Through the above steps, the electrode of the power storage device canbe formed.

Embodiment 2

In this embodiment, an electrode of a power storage device which is oneembodiment of the present invention and a method for manufacturing theelectrode will be described with reference to FIG. 3 and FIGS. 4A and4B.

FIG. 3 illustrates one embodiment of an electrode of a power storagedevice. The electrode of the power storage device in FIG. 3 includes acurrent collector 201, an active material layer 203 provided over onesurface of the current collector 201, and a layer containing niobium 209provided over the active material layer 203. Note that the activematerial layer 203 includes a crystalline silicon region and awhisker-like crystalline silicon region formed over the crystallinesilicon region.

Next, a method for forming the above electrode will be described withreference to FIGS. 4A and 4B.

First, as illustrated in FIG. 4A, a crystalline silicon layer is formedas the active material layer 203 over the current collector 201 by anLPCVD method. Deposition of crystalline silicon by an LPCVD method ispreferably performed at a temperature higher than or equal to 550° C.and lower than or equal to upper temperature limits of an LPCVDapparatus and the current collector 201, further preferably higher thanor equal to 580° C. and lower than or equal to 650° C. As a source gas,a deposition gas containing silicon can be used. Examples of thedeposition gas containing silicon include silicon hydride, siliconfluoride, and silicon chloride; typically, SiH₄, Si₂H₆, SiF₄, SiCl₄,Si₂Cl₆, and the like are given. Note that one or more of rare gases suchas helium, neon, argon, and xenon, nitrogen, and hydrogen may be mixedin the source gas.

For the current collector 201, any of the materials listed above as amaterial for the current collector 101 can be used as appropriate.

Note that oxygen may be contained as an impurity in the active materiallayer 203. This is because oxygen is desorbed from a quartz chamber ofthe LPCVD apparatus by heating performed in the formation of thecrystalline silicon layer as the active material layer 203 by the LPCVDmethod, and the oxygen diffuses into the crystalline silicon layer.

Note that an impurity element that generates a carrier, such asphosphorus or boron, may be added to the crystalline silicon layer. Sucha crystalline silicon layer to which the impurity element that generatesa carrier, such as phosphorus or boron, is added has higherconductivity, so that the conductivity of the electrode can beincreased. Therefore, the discharge capacity or the charge capacity canbe further increased.

The active material layer 203 includes a crystalline silicon region 203a and a whisker-like crystalline silicon region 203 b formed over thecrystalline silicon region 203 a. Note that the interface between thecrystalline silicon region 203 a and the whisker-like crystallinesilicon region 203 b is not clear. Therefore, a plane that is at thesame level as the bottom of the deepest valley among valleys formedbetween plural protrusions in the whisker-like crystalline siliconregion 203 b and is parallel to a surface of the current collector isregarded as the interface between the crystalline silicon region 203 aand the whisker-like crystalline silicon region 203 b.

The crystalline silicon region 203 a is provided so as to cover thecurrent collector 201. The whisker-like crystalline silicon region 203 bhas a plurality of protrusions which projects from unspecified regionsof the crystalline silicon region 203 a in unspecified directions.

Note that the plurality of protrusions in the whisker-like crystallinesilicon region 203 b may each have a columnar shape such as a cylindershape or a prism shape, or a needle shape such as a cone shape or apyramid shape. The top of the protrusion may be curved. The plurality ofprotrusions may include both a columnar protrusion and a needle-likeprotrusion. Further, a surface of the protrusion may be uneven. Thesurface unevenness can increase the surface area of the active materiallayer.

In the electrode of the power storage device described in thisembodiment, the crystalline silicon layer functioning as the activematerial layer 203 includes the whisker-like crystalline silicon region203 b; therefore, the surface area is increased and thus the dischargecapacity or charge capacity of the power storage device at a highcurrent density can be increased.

Next, as illustrated in FIG. 4B, the layer containing niobium 209 isformed over the active material layer 203. The layer containing niobium209 can be formed in a manner similar to that of the layer containingniobium 109 in Embodiment 1.

Through the above steps, the electrode of the power storage device canbe formed.

Embodiment 3

In this embodiment, an electrode of a power storage device which is oneembodiment of the present invention and a method for manufacturing theelectrode will be described below.

First, an active material, a conduction auxiliary agent, a binder, and asolvent are mixed to form a slurry. The slurry is prepared in such amanner that the conduction auxiliary agent is dispersed in the solventcontaining the binder and then the active material is mixed therein. Atthis time, in order to improve the dispersion property, it is preferableto reduce the amount of the solvent so that a thick paste is obtained.After that, the solvent is added and the slurry is formed. Theproportions of the active material, the conduction auxiliary agent, thebinder, and the solvent can be adjusted as appropriate; the higher theproportions of the conduction auxiliary agent and the binder are, thehigher the battery performance per the amount of the active material canbe.

As the active material, a material that can be alloyed with lithium ispreferably used; for example, a material containing silicon, tin,aluminum, or germanium can be used. In this embodiment, granular siliconis used. Note that favorable capacity and cycle characteristics can beobtained when the grain diameter of the granular silicon serving as theactive material is small, and the grain diameter is preferably 100 nm orless.

As the conduction auxiliary agent, a material which is itself anelectron conductor and does not cause chemical reaction with othermaterials in the battery device may be used. For example, carbon-basedmaterials such as graphite, carbon fiber, carbon black, acetylene black,ketjenhlack, and VGCF (registered trademark); metal materials such ascopper, nickel, aluminum, and silver; and powder, fiber, and the like ofmixtures thereof can be given. The conduction auxiliary agent is amaterial that assists conduction between active materials; it fills aspace between active materials which are apart and makes conductionbetween the active materials.

As the hinder, a polysaccharide, a thermoplastic resin, a polymer withrubber elasticity, and the like can be given. Examples thereof includestarch, polyimide, polyvinyl alcohol, carboxymethyl cellulose,hydroxypropyl cellulose, regenerated cellulose, diacetyl cellulose,polyvinyl chloride, polyvinyl pyrrolidone, polytetrafluoroethylene,polyvinylidene fluoride, polyethylene, polypropylene,ethylene-propylene-diene monomer (EPDM), sulfonated EPDM,styrene-butadiene rubber (SBR), butadiene rubber, fluororubber, andpolyethylene oxide.

As the solvent, water, N-methyl-2-pyrrolidone, lactic acid ester, or thelike can be used.

Next, the slurry formed above is applied over a current collector anddried using a hot plate, an oven, or the like. The drying can beperformed at approximately 50° C. in the case of using an aqueous bindersuch as SBR. In the case of using an organic hinder such as PVDF orpolyimide, the drying is preferably performed at approximately 120° C.After that, punching is performed so that a desired shape is obtained,and main drying is performed. The main drying is preferably performed atapproximately 170° C. for approximately 10 hours.

As the current collector, for example, a copper foil, a titanium foil,or a stainless steel foil can be used. In addition, the currentcollector can have a foil shape, a plate shape, a net shape, or the likeas appropriate.

A layer containing niobium is formed over a coated electrode obtainedthrough the above steps. The layer containing niobium can be formed byan evaporation method, for example, and is preferably formed usingniobium oxide or niobium nitride.

Through the above steps, the electrode of the power storage device canbe formed.

Embodiment 4

In this embodiment, a structure of a power storage device will bedescribed with reference to FIGS. 5A and 5B.

First, a structure of a secondary battery will be described below as apower storage device. Here, a structure of a lithium ion battery, whichis a typical example of the secondary battery, will be described.

FIG. 5A is a plan view of a power storage device 151, and FIG. 5B is across-sectional view taken along dashed-dotted line A-H in FIG. 5A. Inthis embodiment, a sealed thin power storage device is described as thepower storage device 151.

The power storage device 151 illustrated in FIG. 5A includes a powerstorage cell 155 in an exterior member 153. The power storage device 151further includes terminal portions 157 and 159 which are connected tothe power storage cell 155. As the exterior member 153, a laminate film,a polymer film, a metal film, a metal case, a plastic case, or the likecan be used.

As illustrated in FIG. 5B, the power storage cell 155 includes anegative electrode 163, a positive electrode 165, a separator 167provided between the negative electrode 163 and the positive electrode165, and an electrolyte 169.

The negative electrode 163 includes a negative electrode currentcollector 171 and a negative electrode active material layer 173.Further, the negative electrode active material layer 173 is formed onone or both of the surfaces of the negative electrode current collector171.

The positive electrode 165 includes a positive electrode currentcollector 175 and a positive electrode active material layer 177.Further, the positive electrode active material layer 177 is formed onone or both of the surfaces of the positive electrode current collector175.

The negative electrode collector 171 is connected to the terminalportion 159. The positive electrode collector 175 is connected to theterminal portion 157. Further, the terminal portions 157 and 159 eachpartly extend outside the exterior member 153.

Note that although a sealed thin power storage device is described asthe power storage device 151 in this embodiment, the power storagedevice can have a variety of structures; for example, a button powerstorage device, a cylindrical power storage device, or a rectangularpower storage device can be used. Further, although the structure inwhich the positive electrode, the negative electrode, and the separatorare stacked is described in this embodiment, a structure in which thepositive electrode, the negative electrode, and the separator are rolledmay be employed.

As the negative electrode current collector 171, the current collector101 described in Embodiment 1 can be used.

For the negative electrode active material layer 173, phosphorus-dopedamorphous silicon can be used as in the case of the active materiallayer 103 described in Embodiment 1. Note that silicon may be pre-dopedwith lithium. In addition, by forming the active material layer 103,which is formed using silicon, with the negative electrode currentcollector 171 held by a frame-like susceptor in an LPCVD apparatus, theactive material layer 103 can be formed on both of the surfaces of thenegative electrode current collector 171 at the same time; thus, thenumber of steps can be reduced.

Further, as in Embodiment 1, a layer containing niobium 179 is formed onthe negative electrode active material layer 173. The layer containingniobium 179 can be formed using niobium oxide or niobium nitride.

Aluminum, stainless steel, or the like is used for the positiveelectrode current collector 175. The positive electrode currentcollector 175 can have a foil shape, a plate shape, a net shape, a filmshape, or the like as appropriate.

For the positive electrode active material layer 177, a material thatoccludes and releases an ion which carries electric charge can be used.For example, the positive electrode active material layer 177 can beformed using LiFeO₂, LiCoO₂, LiNiO₂, LiMn₂O₄, LiFePO₄, LiCoPO₄, LiNiPO₄,LiMn₂PO₄, V₂O₅, Cr₂O₅, MnO₂, or another lithium compound as a material.Note that when carrier ions are alkali metal ions other than lithiumions or alkaline earth metal ions, the positive electrode activematerial layer 177 can be formed using, instead of lithium in the abovelithium compounds, an alkali metal (e.g., sodium or potassium) or analkaline earth metal (e.g., calcium, strontium, or barium) can be used.

As a solute of the electrolyte 169, a material in which lithium ions,i.e., carrier ions can transfer and stably exist is used. Typicalexamples of the solute of the electrolyte include lithium salt such asLiClO₄, LiAsF₆, LiBr₄, LiPF₆, and Li(C₂F₅SO₂)₂N. Note that when carrierions are alkali metal ions other than lithium ions or alkaline earthmetal ions, the solute of the electrolyte 169 can be formed using alkalimetal salt such as sodium salt or potassium salt; alkaline earth metalsalt such as calcium salt, strontium salt, or barium salt; berylliumsalt; magnesium salt; or the like, as appropriate.

As a solvent of the electrolyte 169, a material which can transferlithium ions (or other carrier ions) is used. As the solvent of theelectrolyte 169, an aprotic organic solvent is preferably used. Typicalexamples of the aprotic organic solvent include ethylene carbonate,propylene carbonate, dimethyl carbonate, diethyl carbonate,γ-butyrolactone, acetonitrile, dimethoxyethane, and tetrahydrofuran, andone or more of these materials can be used. When a gelled polymer isused as the solvent of the electrolyte 169, safety against liquidleakage or the like is increased. In addition, the power storage device151 can be thin and lightweight. Typical examples of the gelled polymerinclude a silicon gel, an acrylic gel, an acrylonitrile gel,polyethylene oxide, polypropylene oxide, and a fluorine-based polymer.

As the electrolyte 169, a solid electrolyte such as Li₃PO₄ can be used.Further, the electrolyte 169 may contain niobium. The electrolyte 169may contain vinylene carbonate or the like.

An insulating porous material is used for the separator 167. Typicalexamples of the separator 167 include cellulose (paper), polyethylene,polypropylene, and glass fiber. A single layer or a stacked layer of anyof these materials can be used.

A lithium ion battery has small memory effect, high energy density, andhigh discharge capacity. In addition, the output voltage of the lithiumion battery is high. For those reasons, the size and weight of thelithium ion battery can be reduced. Further, the lithium ion batterydoes not easily deteriorate owing to repetitive charge and discharge andcan be used for a long time, so that cost can be reduced.

Next, a capacitor will be described as a power storage device. Typicalexamples of the capacitor include an electric double-layer capacitor anda lithium ion capacitor.

In the case of a capacitor, instead of the positive electrode activematerial layer 177 in the secondary battery illustrated in FIG. 5B, amaterial capable of reversibly occluding at least one of lithium ions(or other carrier ions) and anions may be used. Typically, the positiveelectrode active material layer 177 can be formed using active carbon, aconductive polymer, or a polyarene organic semiconductor (PAS).

The lithium ion capacitor has high efficiency of charge and discharge,capability of rapidly performing charge and discharge, and a long lifeto withstand repeated use.

By using the negative electrode described in Embodiment 1 as thenegative electrode 163, a power storage device having high dischargecapacity or charge capacity and improved cycle characteristics can bemanufactured.

In addition, by using the current collector and the active materiallayer described in Embodiment 1 in a negative electrode of an air cellwhich is one embodiment of a power storage device, a power storagedevice having high discharge capacity or charge capacity and improvedcycle characteristics can be manufactured.

As described above, in one embodiment of the present invention, amulti-layer structure including a current collector layer, an activematerial layer, a layer containing niobium, and the like can beemployed, whereby substances included in the current collector layer,the active material layer, and the layer containing niobium are bondedto each other and thus the strength of the structure can be increased.Therefore, structural damage due to change in the volume of the activematerial layer along with charge and discharge is less likely to becaused. As a result, even after charge-discharge cycles, damage to theactive material layer is suppressed; accordingly, increase in theresistance inside a battery and decrease in the capacity can besuppressed.

Embodiment 5

In this embodiment, application examples of the power storage devicedescribed in Embodiment 4 will be described with reference to FIGS. 6Ato 6D.

The power storage device described in Embodiment 4 can be used inelectronic devices such as cameras like digital cameras or videocameras, digital photo frames, mobile phones (also referred to ascellular phones or cellular phone devices), portable game machines,portable digital assistants, and audio reproducing devices. Further, thepower storage device can be used in electric propulsion vehicles such aselectric vehicles, hybrid vehicles, train vehicles, maintenancevehicles, carts, and electric wheelchairs. Here, examples of theelectric propulsion vehicles will be described.

FIG. 6A illustrates a structure of a four-wheeled vehicle 300 which isone of the electric propulsion vehicles. The vehicle 300 is an electricvehicle or a hybrid vehicle. The vehicle 300 is an example in which apower storage device 302 is provided in a bottom portion. In order toclearly show the position of the power storage device 302 in the vehicle300. FIG. 6B shows the outline of the vehicle 300 and the power storagedevice 302 provided in the bottom portion of the vehicle 300. The powerstorage device described in Embodiment 4 can be used as the powerstorage device 302. Charge of the power storage device 302 can beperformed by external power supply using a plug-in technique or awireless power feeding system.

FIG. 6C illustrates a structure of a motorboat 1301 which is one of theelectric propulsion vehicles. FIG. 6C illustrates the case where themotorboat 1301 includes a power storage device 1302 equipped on a sideof the body of the boat. The power storage device described inEmbodiment 4 can be used as the power storage device 1302. Charge of thepower storage device 1302 can be performed by external power supplyusing a plug-in technique or a wireless power feeding system. Forexample, a power feeding device for charging the motorboat 1301 (i.e.,for charging the power storage device 1302) may be equipped at a mooringin a harbor.

FIG. 6D illustrates a structure of an electric wheelchair 1311 which isone of the electric propulsion vehicles. FIG. 61) illustrates the casewhere the electric wheelchair 1311 includes a power storage device 1312provided in a bottom portion. The power storage device described inEmbodiment 4 can be used as the power storage device 1312. Charge of thepower storage device 1312 can be performed by external power supplyusing a plug-in technique or a wireless power feeding system.

Embodiment 6

In this embodiment, an example in which a secondary battery that is anexample of the power storage device according to one embodiment of thepresent invention is used in a wireless power feeding system(hereinafter referred to as an RF power feeding system) will bedescribed with reference to block diagrams of FIG. 7 and FIG. 8. In eachof the block diagrams, independent blocks show elements within a powerreceiving device and a power feeding device, which are classifiedaccording to their functions. However, it may be practically difficultto completely separate the elements according to their functions; insome cases, one element can involve a plurality of functions.

First, an RF power feeding system will be described with reference toFIG. 7.

A power receiving device 600 is an electronic device or an electricpropulsion vehicle which is driven by electric power supplied from apower feeding device 700, and can be applied to another device which isdriven by electric power, as appropriate. Typical examples of theelectronic device include cameras like digital cameras or video cameras,digital photo frames, mobile phones (also referred to as cellular phonesor cellular phone devices), portable game machines, portable digitalassistants, audio reproducing devices, display devices, and computers.Typical examples of the electric propulsion vehicle include electricvehicles, hybrid vehicles, train vehicles, maintenance vehicles, carts,and electric wheelchairs. In addition, the power feeding device 700 hasa function of supplying electric power to the power receiving device600.

In FIG. 7, the power receiving device 600 includes a power receivingdevice portion 601 and a power load portion 610. The power receivingdevice portion 601 includes at least a power receiving device antennacircuit 602, a signal processing circuit 603, and a secondary battery604. The power feeding device 700 includes at least a power feedingdevice antenna circuit 701 and a signal processing circuit 702.

The power receiving device antenna circuit 602 has a function ofreceiving a signal transmitted by the power feeding device antennacircuit 701 or transmitting a signal to the power feeding device antennacircuit 701. The signal processing circuit 603 processes a signalreceived by the power receiving device antenna circuit 602 and controlscharge of the secondary battery 604 and supply of electric power fromthe secondary battery 604 to the power load portion 610. In addition,the signal processing circuit 603 controls operation of the powerreceiving device antenna circuit 602. That is, the signal processingcircuit 603 can control the intensity, the frequency, or the like of asignal transmitted by the power receiving device antenna circuit 602.The power load portion 610 is a driving portion which receives electricpower from the secondary battery 604 and drives the power receivingdevice 600. Typical examples of the power load portion 610 include amotor and a driving circuit. Another device which drives the powerreceiving device by receiving electric power can be used as the powerload portion 610 as appropriate. The power feeding device antennacircuit 701 has a function of transmitting a signal to the powerreceiving device antenna circuit 602 or receiving a signal from thepower receiving device antenna circuit 602. The signal processingcircuit 702 processes a signal received by the power feeding deviceantenna circuit 701. In addition, the signal processing circuit 702controls operation of the power feeding device antenna circuit 701. Thatis, the signal processing circuit 702 can control the intensity, thefrequency, or the like of a signal transmitted by the power feedingdevice antenna circuit 701.

The secondary battery according to one embodiment of the presentinvention is used as the secondary battery 604 included in the powerreceiving device 600 in the RF power feeding system illustrated in FIG.7.

With the use of the secondary battery according to one embodiment of thepresent invention in the RF power feeding system, the discharge capacityor the charge capacity (also referred to as the amount of power storage)can be higher than that of a conventional secondary battery. Therefore,the time interval of the wireless power feeding can be longer (frequentpower feeding can be omitted).

In addition, with the use of the secondary battery according to oneembodiment of the present invention in the RF power feeding system, thepower receiving device 600 can be formed to be compact and lightweightif the discharge capacity or charge capacity with which the power loadportion 610 can be driven is the same as that of a conventionalsecondary battery. Therefore, the total cost can be reduced.

Next, another example of the RF power feeding system will be describedwith reference to FIG. 8.

In FIG. 8, the power receiving device 600 includes the power receivingdevice portion 601 and the power load portion 610. The power receivingdevice portion 601 includes at least the power receiving device antennacircuit 602, the signal processing circuit 603, the secondary battery604, a rectifier circuit 605, a modulation circuit 606, and a powersupply circuit 607. In addition, the power feeding device 700 includesat least the power feeding device antenna circuit 701, the signalprocessing circuit 702, a rectifier circuit 703, a modulation circuit704, a demodulation circuit 705, and an oscillator circuit 706.

The power receiving device antenna circuit 602 has a function ofreceiving a signal transmitted by the power feeding device antennacircuit 701 or transmitting a signal to the power feeding device antennacircuit 701. When the power receiving processing circuit 603 has afunction of processing a signal received by the power receiving deviceantenna circuit 602 and controlling charge of the secondary battery 604and supply of electric power from the secondary battery 604 to the powersupply circuit 607. The power supply circuit 607 has a function ofconverting voltage stored in the secondary battery 604 into voltageneeded for the power load portion 610. The modulation circuit 606 isused when a certain response is transmitted from the power receivingdevice 600 to the power feeding device 700.

With the power supply circuit 607, electric power supplied to the powerload portion 610 can be controlled. Thus, overvoltage application to thepower load portion 610 can be suppressed, and deterioration or breakdownof the power receiving device 600 can be reduced.

In addition, with the modulation circuit 606, a signal can betransmitted from the power receiving device 600 to the power feedingdevice 700. Therefore, when the amount of charged power in the powerreceiving device 600 is judged to reach a certain amount, a signal istransmitted from the power receiving device 600 to the power feedingdevice 700 so that power feeding from the power feeding device 700 tothe power receiving device 600 can be stopped. As a result, thesecondary battery 604 is not fully charged, so that the maximum numberof charge cycles of the secondary battery 604 can be increased.

The power feeding device antenna circuit 701 has a function oftransmitting a signal to the power receiving device antenna circuit 602or receiving a signal from the power receiving device antenna circuit602. When a signal is transmitted to the power receiving device antennacircuit 602, the signal processing circuit 702 generates a signal whichis transmitted to the power receiving device. The oscillator circuit 706is a circuit which generates a signal with a constant frequency. Themodulation circuit 704 has a function of applying voltage to the powerfeeding device antenna circuit 701 in accordance with the signalgenerated by the signal processing circuit 702 and the signal with aconstant frequency generated by the oscillator circuit 706. Thus, asignal is output from the power feeding device antenna circuit 701. Onthe other hand, when a signal is received from the power receivingdevice antenna circuit 602, the rectifier circuit 703 rectifies thereceived signal. From signals rectified by the rectifier circuit 703,the demodulation circuit 705 extracts a signal transmitted from thepower receiving device 600 to the power feeding device 700. The signalprocessing circuit 702 has a function of analyzing the signal extractedby the demodulation circuit 705.

Note that any circuit may be provided between circuits as long as the RFpower feeding can be performed. For example, after the power receivingdevice 600 receives a signal and the rectifier circuit 605 generates DCvoltage, a circuit such as a DC-DC converter or regulator that isprovided in a subsequent stage may generate constant voltage. Thus,overvoltage application to the inside of the power receiving device 600can be suppressed.

The secondary battery according to one embodiment of the presentinvention is used as the secondary battery 604 included in the powerreceiving device 600 in the RF power feeding system illustrated in FIG.8.

With the use of the secondary battery according to one embodiment of thepresent invention in the RF power feeding system, the discharge capacityor the charge capacity can be higher than that of a conventionalsecondary battery. Therefore, the time interval of the wireless powerfeeding can be longer (frequent power feeding can be omitted).

In addition, with the use of the secondary battery according to oneembodiment of the present invention in the RF power feeding system, thepower receiving device 600 can be formed to be compact and lightweightif the discharge capacity or charge capacity with which the power loadportion 610 can be driven is the same as that of a conventionalsecondary battery. Therefore, the total cost can be reduced.

Note that in the case where the secondary battery according to oneembodiment of the present invention is used in the RF power feedingsystem and the power receiving device antenna circuit 602 and thesecondary battery 604 overlap with each other, it is preferable that theimpedance of the power receiving device antenna circuit 602 be notchanged by deformation of the secondary battery 604 due to charge anddischarge of the secondary battery 604 and deformation of an antenna dueto the above deformation. When the impedance of the antenna is changed,in some cases, electric power is not supplied sufficiently. Thesecondary battery 604 may be placed in a battery pack formed using metalor ceramics, for example. Note that in that case, the power receivingdevice antenna circuit 602 and the battery pack are preferably separatedfrom each other by several tens of micrometers or more.

In this embodiment, the charging signal has no limitation on itsfrequency and may have any band of frequency as long as electric powercan be transmitted. For example, the frequency of the charging signalmay be selected from any of 135 kHz in LF band, 13.56 MHz in HF band,900 MHz to 1 GHz in UHF band, and 2.45 GHz in SHF band.

A signal transmission method may be properly selected from variousmethods including an electromagnetic coupling method, an electromagneticinduction method, an electromagnetic resonance method, and a microwavemethod. In order to prevent energy loss due to foreign substancescontaining moisture, such as rain and mud, the electromagnetic inductionmethod or the electromagnetic resonance method using a low frequencyband, specifically, VLF of 3 kHz to 30 kHz, LF of 30 kHz to 300 kHz, MFof 300 kHz to 3 MHz, or HF of 3 MHz to 30 MHz is preferably used.

This embodiment can be implemented in combination with any of the aboveembodiments.

Example 1

In this example, a secondary battery which is one embodiment of thepresent invention will be described. In this example, the secondarybattery which is one embodiment of the present invention and a secondarybattery for comparison (hereinafter referred to as a comparativesecondary battery) were manufactured and their battery characteristicswere compared.

(Step of Manufacturing Electrode of Secondary Battery)

First, steps of manufacturing an electrode of the secondary battery willbe described.

An active material layer was formed over a current collector, so thatthe electrode of the secondary battery was manufactured.

As a material for the current collector, titanium was used. As thecurrent collector, a sheet of a titanium foil (also referred to as atitanium sheet) with a thickness of 100 μm was used.

As the active material layer, a phosphorus-doped amorphous silicon layerwas used.

The phosphorus-doped amorphous silicon layer serving as the activematerial layer was formed over the titanium foil serving as the currentcollector by a plasma CVD method. The phosphorus-doped amorphous siliconlayer was formed by a plasma CVD method under the following conditions:silane and phosphine were used as a source gas; silane and phosphinewere introduced into a reaction chamber at flow rates of 60 sccm and 110sccm, respectively; the pressure in the reaction chamber was 133 Pa; andas for the temperature in the reaction chamber, an upper heater was setto 400° C. and a lower heater was set to 500° C.

The phosphorus-doped amorphous silicon layer obtained in the above stepwas used as the active material layer of the secondary battery.

Next, a niobium oxide layer was formed over the formed active materiallayer by an evaporation method. The evaporation was performed in vacuumwith the use of niobium oxide having a composition of Nb₂O₅ as anevaporation source. Further, the composition of the niobium oxide layerwas measured by X-ray photoelectron spectroscopy (XPS). From the result,it was confirmed that the composition of the formed niobium oxide layerwas substantially the same as the composition of the evaporation source,Nb₂O₅.

Through the above steps, the electrode of the secondary battery wasmanufactured.

(Step of Manufacturing Secondary Battery)

Next, steps of manufacturing the secondary battery of this example willbe described.

The secondary battery was manufactured using the electrode obtainedthrough the above steps. Here, a coin-type secondary battery wasmanufactured. A method for manufacturing the coin-type secondary batterywill be described below with reference to FIG. 9.

As illustrated in FIG. 9, the coin-type secondary battery includes anelectrode 204, a reference electrode 232, a separator 210, anelectrolyte (not illustrated), a housing 206, and a housing 244.Besides, the coin-type secondary battery includes a ring-shapedinsulator 220, a spacer 240, and a washer 242. As the electrode 204, anelectrode obtained through the above steps, in which an active materiallayer and a layer containing niobium were provided over a currentcollector, was used. In this example, a titanium foil was used as thecurrent collector, and the active material layer had the stacked-layerstructure including the phosphorus-doped amorphous silicon layer and theniobium oxide layer, which is described in Embodiment 1. The referenceelectrode 232 was formed using lithium metal (a lithium foil). Theseparator 210 was formed using polypropylene. The housing 206, thehousing 244, the spacer 240, and the washer 242 each of which was madeusing stainless steel were used. The housing 206 and the housing 244have a function of making external electrical connection of theelectrode 204 and the reference electrode 232.

The electrode 204, the reference electrode 232, and the separator 210were soaked in the electrolyte. Then, as illustrated in FIG. 9, thehousing 206, the electrode 204, the separator 210, the ring-shapedinsulator 220, the reference electrode 232, the spacer 240, the washer242, and the housing 244 were stacked in this order so that the housing206 was positioned at the bottom. The housing 206 and the housing 244were pressed and crimped to each other with a “coin cell crimper”. Insuch a manner, the coin-type secondary battery was manufactured.

The electrolyte in which LiPF₆ was dissolved in a mixed solvent ofethylene carbonate (EC) and diethyl carbonate (DEC) was used.

(Step of Manufacturing Electrode of Comparative Secondary Battery)

Next, steps of manufacturing an electrode of the comparative secondarybattery will be described. A step of forming an active material layer ofthe comparative secondary battery is different from that of thesecondary battery which is one embodiment of the present invention. Theother structures of the comparative secondary battery are the same asthose of the secondary battery which is one embodiment of the presentinvention; therefore, description of structures of a substrate, acurrent collector, and the like is omitted.

The active material layer of the comparative secondary battery had asingle-layer structure of a phosphorus-doped amorphous silicon layer.

(Step of Manufacturing Comparative Secondary Battery)

Next, steps of manufacturing the comparative secondary battery will bedescribed.

The active material layer was formed over a current collector in theabove-described manner, so that the electrode of the comparativesecondary battery was formed. The comparative secondary battery wasmanufactured using the electrode. The comparative secondary battery wasmanufactured in the same manner as the above secondary battery exceptfor the formation method of the electrode.

(Characteristics Comparison Between Secondary Battery of the PresentInvention and Comparative Secondary Battery)

Battery characteristics of the secondary battery and the comparativesecondary battery which were manufactured in the above-described mannerwere compared using a charge-discharge measuring instrument. For themeasurements of charge and discharge, a constant current mode was used,and charge and discharge were performed with a current of 0.05 mA onlyin initial charge and 0.15 mA in charge after the initial charge. Theupper limit voltage was 1.0 V, and the lower limit voltage was 0.03 V.The capacity limit was 2000 (mAh/g), and the measurements were conductedat room temperature. The results are shown in Table 1 and FIG. 10.

TABLE 1 Charge-discharge efficiency Values in parentheses represent thedischarge capacity of lithium ion (mAh/g) After 2 After 10 After 20After 30 cycles cycles cycles cycles Comparative 96.0% 98.3% 98.0% 93.1%secondary battery (1920) (1966) (1962) (1500) Secondary battery of  100% 100%  100%  100% the present invention (2000) (2000) (2000) (2000)

Table 1 shows the proportion of the amount of released lithium ion tothe amount of occluded lithium ion at varied charge-discharge cyclenumbers; in other words, Table 1 shows results of evaluation of thecharge-discharge efficiency. From these results, it was found that thesecondary battery which included the electrode obtained by depositingniobium oxide over the active material layer had much highercharge-discharge efficiency than the comparative secondary battery whichincluded the electrode obtained without depositing niobium oxide overthe active material layer, and had an irreversible capacity ofsubstantially 0. Note that the discharge capacity (mAh/g) was calculatedon the assumption that the weight of each of the active material layersof the secondary battery and the comparative secondary battery was 0.255mg.

FIG. 10 shows measurement results of the amount of released lithium ionwith respect to the number of charge-discharge cycles. From theseresults, it was found that the lithium-ion discharge capacity of thesecondary battery which included the electrode obtained by depositingniobium oxide over the active material layer was not decreased even whenthe number of charge-discharge cycles was increased, in contrast withthat of the comparative secondary battery which included the electrodeobtained without depositing niobium oxide over the active materiallayer.

From the results in Table 1 and FIG. 10, it was found that thecharge-discharge efficiency and cycle characteristics of the secondarybattery which included the electrode obtained by depositing niobiumoxide over the active material layer were improved as compared with thecomparative secondary battery which included the electrode obtainedwithout depositing niobium oxide over the active material layer.

EXPLANATION OF REFERENCE

101: current collector, 103: active material layer, 109: layercontaining niobium, 151: power storage device, 153: exterior member,155: power storage cell, 157: terminal portion, 159: terminal portion,163: negative electrode, 165: positive electrode, 167: separator, 169:electrolyte, 171: negative electrode current collector, 173: negativeelectrode active material layer, 175: positive electrode currentcollector, 177: positive electrode active material layer, 179: layercontaining niobium, 201: current collector, 203: active material layer,203 a: crystalline silicon region, 203 b: crystalline silicon region,204: electrode, 206: housing, 209: layer containing niobium, 210:separator, 220: ring-shaped insulator, 232: reference electrode, 240:spacer, 242: washer, 244: housing, 300: vehicle, 302: power storagedevice, 600: power receiving device, 601: power receiving deviceportion, 602: power receiving device antenna circuit, 603: signalprocessing circuit, 604: secondary battery, 605: rectifier circuit, 606:modulation circuit, 607: power supply circuit, 610: power load portion,700: power feeding device, 701: power feeding device antenna circuit,702: signal processing circuit, 703: rectifier circuit, 704: modulationcircuit, 705: demodulation circuit, 706: oscillator circuit, 1301:motorboat, 1302: power storage device, 1311: electric wheelchair, and1312: power storage device.

This application is based on Japanese Patent Application serial no.2010-272903 filed with the Japan Patent Office on Dec. 7, 2010, theentire contents of which are hereby incorporated by reference.

1. A power storage device comprising: an electrode comprising: a currentcollector; an active material layer formed over the current collectorand comprising a material that can be alloyed with lithium; and a layercontaining niobium, formed over the active material layer.
 2. The powerstorage device according to claim 1, wherein the layer containingniobium comprises niobium oxide or niobium nitride.
 3. The power storagedevice according to claim 2, wherein the layer containing niobiumfurther comprises niobium-lithium alloy.
 4. The power storage deviceaccording to claim 1, wherein the material that can be alloyed withlithium is silicon, tin, aluminum, or germanium.
 5. The power storagedevice according to claim 1, wherein the material that can be alloyedwith lithium is silicon including phosphorus or boron.
 6. The powerstorage device according to claim 1, wherein the active material layercomprises a crystalline silicon region and a whisker-like crystallinesilicon region over the crystalline silicon region.
 7. A power storagedevice comprising: a negative electrode comprising: a current collector;an active material layer formed over the current collector andcomprising a material that can be alloyed with lithium; and a layercontaining niobium, formed over the active material layer; anelectrolyte formed in contact with the negative electrode; and apositive electrode facing the negative electrode with the electrolytepositioned between the positive electrode and the negative electrode. 8.The power storage device according to claim 7, wherein the layercontaining niobium comprises niobium oxide or niobium nitride.
 9. Thepower storage device according to claim 8, wherein the layer containingniobium further comprises niobium-lithium alloy.
 10. The power storagedevice according to claim 7, wherein the material that can be alloyedwith lithium is silicon, tin, aluminum, or germanium.
 11. The powerstorage device according to claim 7, wherein the material that can bealloyed with lithium is silicon including phosphorus or boron.
 12. Thepower storage device according to claim 7, wherein the electrolyteincludes niobium.
 13. An electrode of a power storage device comprising:a current collector; an active material layer formed over the currentcollector and comprising a material that can be alloyed with lithium;and a layer containing niobium, formed over the active material layer.14. The electrode of the power storage device according to claim 13,wherein the current collector includes at least one element selectedfrom the group consisting of copper, platinum, aluminum, nickel,tungsten, molybdenum, titanium, iron, silicon, neodymium, scandium,zirconium, hafnium, vanadium, niobium, tantalum, chromium, cobalt,indium, zinc, and tin.
 15. The electrode of the power storage deviceaccording to claim 13, wherein the layer containing niobium comprisesniobium oxide or niobium nitride.
 16. The electrode of the power storagedevice according to claim 15, wherein the layer containing niobiumfurther comprises niobium-lithium alloy
 17. The electrode of the powerstorage device according to claim 13, wherein the material that can bealloyed with lithium is silicon, tin, aluminum, or germanium.
 18. Theelectrode of the power storage device according to claim 13, wherein thematerial that can be alloyed with lithium is silicon includingphosphorus or boron.
 19. An electrode of a power storage devicecomprising: a current collector; an active material layer formed overthe current collector and comprising a material that can be alloyed withlithium; and a layer formed over the active material layer, wherein thelayer formed over the active material layer includes at least oneelement selected from the group consisting of vanadium, tantalum,tungsten, zirconium, molybdenum, hafnium, chromium, and titanium.